To allow for multiple row inserts, you'd do something like:
CREATE TABLE Source (srcID int IDENTITY, srcField1 nvarchar(50))
CREATE TABLE Destination (destID int IDENTITY, destField1 nvarchar(50))
CREATE TRIGGER tr_SourceInsert ON [dbo].[Source]
FOR INSERT AS
SET d.Field1 = i.Field1
FROM inserted AS i
JOIN destination AS d
ON d.destID = '1'
INSERT Source(srcfield1) VALUES ('1')
INSERT Source(srcfield1) VALUES ('A')
This code still seems suspect for a trigger, since I can't fathom why you would always want to update the destination table this way. There may be a reason for it, but I'm skeptical.